Three main types of object detection apparatuses, i.e. a stereo image sensor, a laser radar, and a millimeter wave radar, are currently in practical use for detecting an object at a remove of several meters or more.
The advantages of a stereo image sensor are that the observation subject space is large, the bearing of the object is accurate, object identification is easy, and so on. The disadvantages are that the precision of the distance to the object is low, the sensor is affected easily by its peripheral environment, such as rain or fog, and so on.
The advantages of a laser radar are that it is small, the precision of the distance to the object is high, and so on. The disadvantages are that the observation subject space is small, the radar is affected easily by its peripheral environment, such as rain or fog, and so on.
The advantages of a millimeter wave radar is that the precision of the distance to the object is high, the radar is not affected easily by its peripheral environment, such as rain or fog, and so on. The disadvantages are that the observation subject space is small, object identification is difficult, and so on.
The performance requirements of an object detection apparatus include a large observation subject space, accurate detection of only the object to be detected, accurate detection of the three-dimensional position of the object, and so on, and the apparatus is also required to have a small size. However, it is difficult to achieve these performance requirements with a stand-alone image sensor, laser radar, or millimeter wave radar due to the respective disadvantages thereof described above.
To compensate for these disadvantages, an object detection apparatus combining a stereo image sensor and a laser radar (see Japanese Unexamined Patent Application Publication H6-59033 (pages 3 to 7, FIG. 1), for example) and an object detection apparatus combining a stereo image sensor and a millimeter wave radar (see Japanese Unexamined Patent Application Publication 2001-296357 (pages 3 to 5, FIG. 6), for example) have been proposed.
However, a combination of a stereo image sensor and a laser radar is also affected easily by the peripheral environment, and it is therefore difficult to obtain a stable performance.
Moreover, if a conventional millimeter wave radar is used in a combination of a stereo image sensor and a millimeter wave radar, the size of the antenna cannot be reduced, and it is therefore difficult to reduce the size of the apparatus. A conventional millimeter wave radar detects the bearing of an object, the distance to the object, and the relative velocity of the object by reducing the beam width of the emitted millimeter waves by an angle of approximately two to four degrees, and varying the direction in which the beam is emitted mechanically or electronically. The beam width must be reduced to increase the bearing resolution, and hence the antenna size and beam width are in inverse proportion, making it impossible to reduce the size of the antenna in order to improve the bearing resolution.